1. Field of the Invention
This invention relates generally to tomography utilizing a nuclear magnetic resonance (NMR) phenomenon, and more particularly to a technique of imaging a blood flow inside a body.
2. Description of the Prior Art
The principle of blood flow imaging lies in that a gradient magnetic field which does not exert any influences upon a stationary object but does only upon a moving object is applied in a flow direction and measures a flow speed by adding phase information that varies in accordance with the flow speed. FIG. 1 of the accompanying drawings illustrates this principle. It will now be assumed that a blood flow flows through a blood vesel 11 in a Z direction. A gradient field 12 (G.sub.z) is applied at a time .tau..sub.1 and an inversion gradient field 13 (-G.sub.z) is applied at a time .tau..sub.2 after .DELTA..tau. from the time .tau..sub.1. The inversion gradient field has the same magnitude as the gradient field (G.sub.Z) but its polarity is opposite. It is called a bi-polar gradient.
Since a stationary object does not have any motion, it feels the magnetic fields that have the same magnitude but reversed polarities at the time .tau..sub.1 and .tau..sub.2, and the influences of these fields are cancelled with each other so that the state at this time is exactly the same as the state where no gradient field is applied at all. On the other hand, since a blood flow portion has motion, it feels different fields at the time .tau..sub.1 and .tau..sub.2, and the influences of these fields are not cancelled but provide a phase change to spin.
The following relation is established between the phase and the flow speed with t.sub.P and t.sub.I representing time and interval of the bi-polar gradient field, respectively: EQU .theta.=0.36.nu..sub.o VG.sub.z t.sub.P t.sub.I ( 1)
where .theta.: phase rotation angle (degree) due to flow speed,
.nu..sub.o : resonance frequency (4.258 KHz/G), PA0 V: flow speed (cm/sec), PA0 G.sub.Z : gradient of gradient field (G/cm), PA0 t.sub.P : time of bi-polar gradient field (msec), PA0 t.sub.I : interval of bi-polar gradient field (msec). PA0 G.sub.x : gradient of gradient field in x direction PA0 V: flow speed PA0 t.sub.P : G.sub.x basic time PA0 t.sub.I : G.sub.x interval.
In other words, the phase angle is proportional to the flow speed V, the time t.sub.P of the gradient field and the interval of the gradient field. Since t.sub.I and t.sub.P are constant at the time of imaging, the phase angle is after all proportional to the flow speed and its value can be set arbitrarily by controlling t.sub.P and t.sub.I. A sequence that provides phase information to motion by the combination of two gradinet fields having such t.sub.I and t.sub.P is referred to as a "flow encoded pulse".
Conventionally, imaging has been effected by adding this flow encoded pulse to an ordinary sequence. In this case, thephase changes due to the following factors in addition to the flow encoded pulse:
(1) phase change resulting from distortion of the apparatus such as distortion of a stationary magnetic field; and
(2) phase change due to the influences of a slice selection gradient field, which functions equivalently to the flow encoded pulse contained in the ordinary sequence, and of a read-out gradient field (they affect only a moving object).
The flow speed cannot be measured accurately unless these phase change components are removed. Therefore, it has been customary in the art to effect imaging in the ordinary sequence with the addition of the flow encoded pulse and then to measure the flow speed from the phase of its difference.
In the case of the phase, however, the method based upon the difference described above involves the following problems. Namely, since the phase has a cyclic value for every 2.pi., the phase cannot be determined accurately if a phase angle becomes great. This means that a dynamic range of measurement becomes narrow. Particularly, a critical problem lies in that a portion which is dependent upon the flow speed exists among uncontrollable phase components.
The method relying upon the difference must effect imaging at least twice.
Next, the problems of the prior art technique encountered in blood flow measuring using the flow encoded pulse will be described.
Generally, a slice vertical direction, an image lateral direction as a direction of the read-out gradient field and an image longitudinal direction as the phase encoding direction are referred to as Z direction, x direction and y direction, respectively. Hereinafter, the description will be made by use of these z, x and y directions.
The combination or a set of the two gradient fields described above is referred to as the flow encoded pulse and is always used for the measurement of the blood flow. However, its influence varies depending upon whether the direction of the blood flow is the z direction or the x direction. The reason will be explained with reference to FIG. 2 showing an imaging pulse sequence.
In the drawing, three kinds of "flow encoded pulses" exist. Namely, two kinds of G.sub.z and one for G.sub.x. As to G.sub.z, one set of pulses 102 and one set of pulses 105 exist. Though only one pulse 105 is shown in the dawing, a 180.degree. pulse 104 which is applied simultaneously functions substantially in the same way as the pulse 105. Two phase rotations occur due to the gradient fields by the pulses 102 and 105 on the basis of the principle described above.
The pulses 103 and 107 are the flow encoded pulses for G.sub.x. Unlike the z direction, the field is a gradient field during the read-out operation of a measuring signal 108. Therefore, the blood flow in the x direction changes the frequency during the measurement. Though individual spins exhibit complicated behaviours, the phase rotation proportional to the flow speed occurs in the same way as in the z direction.
The following methods have conventionally been reported as to the measurement of the blood flow in the x direction. The first method is described in "Phase Encoded NMR Flow Imaging" by D. G. Norris, pages 559-560 proceeding published by Society of Magnetic Resonance in Medicine, third annual meetings, 1984. A flow encoding is added in the same way as in the z direction at times other than at the time of measurement. Imaging is effected eight times while changing the conditions and an image of speed resolution of eight points can be obtained by Fourier Transform in that direction.
The second method is described in "NMR Velocity Imaging by Phase Display" by V. J. Wedeen, pages 742-743, proceedings published by Society of Magnetic Resonance in Medicine, third annual meetings, 1984. This method uses directly the pulses 103 and 107 for G.sub.x, but in order to examine the speed change, the position of the pulse 107 is deviated so that the phase changes linearly with respect to the position. Since the phase changes cyclically for every 2.pi., the resulting image appears as a fringe-like pattern. Though the fringe pattern is a clear longitudinal fringe in the case of a stationary object, it is distorted in accordance with the magnitude of motion of the object if any motion exists. The flow speed can be observed from this as a pattern.
These two methods described above are qualitative imaging methods and hence, involve quantitative problems.
In addition, the number of images taken must be increased in accordance with the first method in order to improve speed resolution, and this is a critical problem in NMR imaging which requires an elongated imaging time. In accordance with the second method, on the other hand, the z direction cannot be distinguished from the x direction if any motion of the object exists in the z direction. For this reason, this method is limited only to the case where the motion exists only inside the y plane, and is not free from a large practical problem.